Acceleration behavior of flame spread over fully enclosed noise barriers

Fully enclosed noise barriers have been increasingly used in transportation infrastructure, yet their flame spread behavior remains unclear. In this study, flame spread experiments were conducted on 5 mm thick polymethyl methacrylate (PMMA) sheets in a fully enclosed noise barrier configuration with different sample lengths ( L = 25−100 cm) and widths ( W = 10.0−25.0 cm). The results show that the average flame spread rate (AFSR) increases with both sample width and sample length, while the effect of width is more pronounced. A predictive model of AFSR under the coupled effects of length and width is further established and agrees well with the experimental results. In addition, flame spread over the fully enclosed noise barrier exhibits clear acceleration behavior and undergoes three stages: (I) the unilateral flame spread stage, (II) the counter-directional flame spread stage, and (III) the merging combustion stage. The transition from Stage I to Stage II is accompanied by evident flame acceleration. When W < 15 cm, the flame spreads steadily, whereas accelerated flame spread occurs at 15 ≤ W ≤ 25 cm. This acceleration is attributed to enhanced heat transfer by high-temperature pyrolysis gases and smoke beneath the lower surface, which strengthens thermal feedback to the preheating zone. Overall, the results indicate that W is an important factor triggering flame spread acceleration over fully enclosed noise barriers, providing useful preliminary evidence for fire safety design of fully enclosed noise barriers.